The present invention relates to a process for preparing secondary amines from primary amines over transition metal catalysts.
Processes for preparing secondary amines from primary amines are known per se. It is prior art to convert a primary amine having the desired substituents or structural elements into the desired secondary amine under hydrogen and under the reaction conditions chosen in each case. Various catalysts are used; the pressures and temperatures employed in the reaction vary widely. A frequent problem in this synthesis of the secondary amines is that the conversion or the selectivity to the desired product frequently do not achieve the desired values. It is often also necessary to use expensive noble metal catalysts.
DE-A 30 48 832 relates to a process for preparing amines, in particular bis(3-dimethylamino)propylamine (bis-DMAPA) from 3-dimethylaminopropionitrile (DMAPN) or 3-dimethylaminopropylamine (DMAPA) or mixtures of DMAPN and DMAPA. In an example, a bis-DMAPA selectivity of 80% is achieved at a DMAPA conversion of 53% at high pressure (173 bar) over Nixe2x80x94Cuxe2x80x94Cr2O3, while a bis-DMAPA selectivity of 88% is achieved at a DMAPA conversion of 49% over Coxe2x80x94Cuxe2x80x94Cr2O3.
It is found here that, in particular, the conversions are much too low. In addition, for environmental reasons, the use of chromium-containing catalysts is no longer acceptable.
It is an object of the present invention to provide a process for preparing secondary amines from primary amines which can be carried out using chromium-free catalysts and gives the desired secondary amines in high yields and selectivities.
We have found that this object is achieved by a process for preparing a secondary amine of the formula 
where
R1, R2 may be identical or different and are each, independently of one another, a linear or branched alkyl radical having from 1 to 20 carbon atoms which may bear from 1 to 5 phenyl groups as substituents or a cyclohexyl radical or together with the N atom to which they are bound form a 3- to 7-membered saturated ring which may contain further heteroatoms selected from the group consisting of N, O and S and may be substituted by from 1 to 5 alkyl groups having 1 or 2 carbon atoms,
A is a linear or branched alkylene group having from 2 to 20 carbon atoms which may have from 1 to 5 phenylene groups in its chain, or a radical of the formula 
xe2x80x83where R3=H or CH3, X=O or S or an NR4 group in which R4 is H or a linear or branched alkyl group having from 1 to 4 carbon atoms, k is 1 or 2 and m is an integer from 0 to 4, or a group of the formula 
where n, o and p, q are each, independently of one another, integers from 1 to 4,
by reaction of primary amines of the formula
R1R2Nxe2x80x94Axe2x80x94NH2xe2x80x83xe2x80x83(II), 
where R1, R2 and A are as defined for formula (I), in the presence of hydrogen and a catalyst comprising at least one element or compound of an element of groups VIII and IB of the Periodic Table of the Elements. In one embodiment of the present invention, the catalyst composition can be free of a metal of group IB or a compound thereof. In a preferred embodiment, the catalyst contains up to 50% by weight of at least one element or compound of an element of group IB of the Periodic Table of the Elements.
The catalysts used in the process of the present invention thus comprise, in the active catalyst composition, up to 100% by weight of at least one element or at least one compound of an element of groups VIII and IB of the Periodic Table of the Elements, i.e. from the group consisting of Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag and Au. In one embodiment, the active catalyst composition can contain 0% by weight, in a preferred embodiment up to 50% by weight, or at least one element or at least one compound of an element of group IB of the Periodic Table of the Elements, i.e. from the group consisting of Cu, Ag and Au, preferably Cu. The amount of metal or compound of a metal of group IB is, in a more preferred embodiment, from 1 to 30% by weight, in particular from 10 to 25% by weight, based on the total amount of active catalyst composition. In a further preferred embodiment, the active catalyst composition comprises at least one element or at least one compound of an element from the group consisting of Ni, Co, Cu, Ru, Rh, Ir, Pd, Pt, in the ratios indicated above for the general and preferred embodiments.
If compounds of the specified metals are used for preparing the catalyst, it is possible to use, for example, the oxides, nitrates, carbonates, chlorides and acetates.
In the most preferred embodiment of the present invention, the oxides of the elements employed are used for preparing the catalyst. These are then reduced before use in the reaction, preferably by treatment with hydrogen. This gives a catalyst in which the metal components employed are present in elemental, finely-divided form.
The catalysts can be used as all-active catalysts or in supported form. When using supported catalysts, the proportion of support is from 10 to 90% by weight, based on the total mass of the catalyst (active composition plus support).
As supports, it is possible to use all known suitable supports, for example, activated carbon, silicon carbide or metal oxides. The use of metal oxides is preferred. Among metal oxides, preference is given to using aluminum oxide, silicon dioxide, titanium dioxide, zirconium dioxide, zinc oxide, magnesium oxide or mixtures thereof, which may, if appropriate, be doped with alkali metal oxides and/or alkaline earth metal oxides. Particular preference is given to xcex3-aluminum oxide, silicon dioxide, zirconium dioxide or titanium dioxide or mixtures thereof, in particular Al2O3. The supports can be used in any form, for example as extrudates (rod form), pellets, tablets, monoliths, woven meshes, knitteds or in powder form. The supported catalysts can be prepared by generally known methods. These include, for instance, impregnation of a support with solutions of compounds of the metal components used. Suitable solvents include all customary solvents, for instance water, methanol, ethanol or acetone; preference is given to using water. Furthermore, the catalyst can be prepared by coprecipitation or sequential precipitation of the catalyst components, followed by filtration and washing of the filter cake. The impregnation or precipitation is followed by a drying step (50-200xc2x0 C.) and a calcination step (200-500xc2x0 C.). The catalysts are then reduced at final temperatures of from 200 to 400xc2x0 C. and can subsequently be passivated, since the reduced metals are pyrophoric. After installation of the catalysts in the synthesis reactor, the catalysts can be reactivated by reduction with hydrogen at from 100 to 300xc2x0 C. before the reaction is started.
According to the present invention, preference is given to the use as starting materials of primary amines of the formula (I) and the synthesis of secondary amines of the formula (II) in which the substituents R1, R2 and A have the following meanings:
A is a linear or branched methylene chain having from 2 to 10 carbon atoms or a group of the formula
xe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94(CH2xe2x80x94CH2xe2x80x94O)nxe2x80x94CH2xe2x80x94CH2xe2x80x94, 
xe2x80x83where n is an integer from 0 to 2,
R1, R2 are identical or different and are each, independently of one another, an alkyl radical having from 1 to 12 carbon atoms or together with the nitrogen atom to which they are bound form a 5- or 6-membered saturated ring which may contain a further heteroatom selected from the group consisting of O and N.
Particular preference is given to amines of the formulae (I) and (II) in which
A is an alkylene group having from 1 to 6 carbon atoms or a group of the formula
xe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94CH2xe2x80x94CH2xe2x80x94
and
R1 and R2 may be identical or different and are each, independently of one another, a linear or branched alkyl radical having from 1 to 4 carbon atoms or together with the nitrogen atom to which they are bound form a piperidine ring or a morpholine ring.
In particular, 3-dimethylaminopropylamine (DMAPA) is used to produce bis-3-dimethylaminopropylamine (bis-DMAPA).
The process of the present invention is carried out at from 50 to 250xc2x0 C., preferably from 90xc2x0 C. to 170xc2x0 C., particularly preferably from 120xc2x0 C. to 160xc2x0 C., at pressures of from 5 to 350 bar, preferably from 5 to 200 bar, particularly preferably from 10 to 100 bar, in particular from 10 to 30 bar, either batchwise or preferably continuously in pressure apparatuses such as autoclaves or preferably in a tube reactor. The pressure is preferably the hydrogen pressure in the reactor. When using a tube reactor, the catalyst used can also be present as a fixed-bed catalyst.
The reaction can be carried out in the gas phase or in the liquid phase.
The space velocity over the catalyst, based on the primary amine used, is preferably from 0.1 to 2 kg 1xe2x88x921hxe2x88x921, in particular from 0.8 to 1.2 kg 1xe2x88x921hxe2x88x921. Part of the liquid reaction product can be recirculated to the reaction.
The process of the present invention can be carried out in the absence of solvents or in solvents such as water, methanol, ethanol, tetrahydrofuran, methyl tert-butyl ether or N-methylpyrrolidone. If a solvent is used, the primary amine employed can be dissolved in the solvent. The solvent can also be fed separately into the reactor at any point. Preference is given to carrying out the process in the absence of a solvent.
The desired secondary amine obtained by means of the process of the present invention can be separated off from the reaction mixture and purified in a manner known per se, for example by distillation.
It is also possible, for example, to obtain a stream comprising pure secondary amine and a stream comprising primary amine by rectification and to recirculate the stream comprising the primary amine to the synthesis.
According to the present invention, the primary amines of the formula (II) and the secondary amines of the formula (I) are preferably obtained in a weight ratio of from 10:1 to 1:10, preferably 2:3-4.
The process of the present invention makes it possible to obtain end product mixtures which contain only small amounts of tertiary amines, generally in amounts of  less than 10% by weight. The process can also be carried out so that  less than 5% by weight of tertiary amines are obtained. Under optimum reaction conditions, it is also possible for no tertiary amines to be formed.
The amines obtainable by means of the process of the present invention, preferably bis-DMAPA, are hardeners for epoxy resins, catalysts for polyurethanes, intermediates for the preparation of quaternary ammonium compounds, plasticizers, corrosion inhibitors, textile assistants, dyes and emulsifiers. Polyfunctional tertiary amines are also employed for producing synthetic resins, ion exchangers, pharmaceuticals, crop protection agents and pesticides.
The invention is illustrated by the following example: